It was hissing right out of the box, it's brand new.
I don't suspect so much that it's defective, I hear a lot of switchers,
it's almost as if there's a couple of popular frequencies, and I happen
to be able to hear one of them. If I knew what section is resonating
(physically vibrating and transducing the sound that is) I could try to
drizzle a bit of super glue on it.

B.




Dave Platt wrote:


I've heard switching PSUs begin to hiss when the electrolytic filter
capacitors start to fail. These caps are in a stressful operating
environment - high ripple currents, and sometimes high temperatures.
They can dry out or leak (this is a particular problem with a whole
batch of Taiwanese-made caps which were manufactured with a defective
electrolyte formula), their ESR increases, and the circuit becomes
audibly noisy. I don't know whether the noise is direct microphonics
from the caps, or from the toroids.

Consider swapping out any suspect 'lytics in the supply with new ones,
preferably ones rated for switching-supply operation at 105 degree C.